Cozing up by the fireplace and letting its warmth lull you to sleep is many people’s ideal of peace and quality rest. But that romantic idyll may hold more weight in theory than in practice.
Besides research, be it still fairly scarce, sleep experts who link scientific dots believe that it’s the cooler temperatures that produce the most restful sleep. Of course, going too cool can have a reverse effect, too.
This is linked to the body’s internal, or core temperature. Throughout the day, it fluctuates between roughly 98.6 to 100.4 degrees. Our core temperature is controlled naturally by the brain. And while people’s core temperatures can vary slightly, there’s a constant that holds true across all humans: low temperature signals the body that it’s time to sleep, while high temperature starts waking it up.
Conversely, our shell temperature, which includes the skin, the subcutaneous (under the skin) tissues, and the limbs, is tied to external environment, like room temperature, and it can affect core temperature, too.
This is why an overly warm bedroom confuses the body, warming it up to seize the day even though it’s night-time.
Define an “Overly Warm” Bedroom
The general rule of thumb is 60 to 67, with some leeway for extra warmth, up until 72, with 65-72 being many people’s Goldilocks range. Anything above 72 and below 60 has been shown to have a negative impact on sleep in studies. Speaking of…
“In real-life situations where bedding and clothing are used, heat exposure increases wakefulness and decreases slow wave sleep and rapid eye movement sleep. Humid heat exposure further increases thermal load during sleep and affects sleep stages and thermoregulation,” say a team of scientists in the Journal of Physiological Anthropology. 
Another study brought sleep apnea into the equation, exploring how 40 untreated sleep apnea patients’ sleep quality, as defined by sleep length and alertness in the morning, was affected by room temperature. The thermal points of reference were three: 60.8, 68, and 75.2 degrees. 
The lowest temperature produced the highest quality sleep, however, the findings came with a little twist.
“Untreated patients with obstructive sleep apnea sleep longer, have better sleep efficiency, and are more alert in the morning after a night’s sleep at 16°C room temperature compared with 24°C, but obstructive sleep apnea is more severe at 16°C and 20°C compared with 24°C.”
Moreover, a sweeping data review from 765,000 U.S. survey respondents established a very telling trend: “increases in nighttime temperatures amplify self-reported nights of insufficient sleep,” which exacerbates “during the summer and among both lower-income and elderly respondents.” 
Finally, cold sleep temperature (66 degrees) was found to increase brown fat volume and fat metabolic activity by 42% and 10%, respectively, as opposed to 75 and 81 degrees.  Brown fat stimulates energy burning, thus protecting against obesity and diabetes.
Keeping your bedroom temperature relatively low – 60-72 degrees, while still using blanket, can go a long way toward improving various facets of your lifestyle, health, and wellness.
1- Mizuno and Mizuno, Effects of thermal environment on sleep and circadian rhythm, Journal of Physiological Anthropology, 31 May 2012, Journal Impact Factor = 2.824, Times Cited = 242
2- Valham et al, Ambient Temperature and Obstructive Sleep Apnea: Effects on Sleep, Sleep Apnea, and Morning Alertness , Sleep, Volume 35, Issue 4, 1 April 2012, Pages 513–517, Journal Impact Factor = 5.135, Times Cited = 26
3- Obradovich et al, Nighttime temperature and human sleep loss in a changing climate, Sci Adv.2017 May, Journal Impact Factor = 14.136, Times Cited = 100
4- Lee et al, Temperature-acclimated brown adipose tissue modulates insulin sensitivity in humans, Diabetes, 2014 Nov;63(11):3686-98, Journal Impact Factor = 7.72; Times Cited = 327